The present Invention relates to an imaging method based on nuclear magnetic resonance for the examination of an object, such as a human or animal body or industrial process.
Magnetic resonance Imaging (MRI) utilizes the nuclear magnetic resonance phenomenon (NMR) to determine the local distributions of the nuclear density and nucleus related NMR properties of an object or the physical and chemical characteristics having an effect thereon. Such NMR properties include longitudinal relaxation time (T1), transverse relaxation time (T2), relaxation time In a rotating frame (T1 rho), chemical shift, and coupling factors between nuclei. NMR properties are effected by physical phenomena such as flow rate, diffusion, paramagnetic materials, ferromagnetic materials, viscosity and temperature.
Methods and applications of magnetic resonance and magnetic resonance imaging have been described In the following references: "Biomedical Magnetic Resonance Imaging", by Wehrli, Shaw, and Kneeland, VCH Publishers Inc.. New York 1988; "Magnetic Resonance Imaging", by Stark and Bradley, C.V. Mosby Company, St Louis 1991; "The principles of nuclear magnetism", by Abragam, Clarendon press, Oxford 1961; "Pulse and Fourier Transform NMR", by Farrar and Becker, Academic Press, New York 1971; and U.S. Pat. No. 4,070,611 to Ernst. Additional references are mentioned In U.S. Pat. No. 5,159,270 to Sepponen.
In U.S. Pat. No. 4,668,904, to Kupiainen, there are described fast, controllable current sources which are needed in MRI experiments.
Imaging sequences having a small excitation angle and the spin echo formed by means of gradient operations are described in U.S. Pat. No. 4,707,658 to Frahm, Haase, Matthaei, Haenicke and Merboldt; U.S. Pat. No. 4,699,148 to Gygnel; "Koelaitteisto NMR-kuvausta varten" (Experimental apparatus for NMR-imaging by Tanttu, Master of Science thesis, Helsinki Technical University, Department of Technical Physics, 1982, page 69; and "Koelaitteisto liikkuvan kohteen NMR-kuvausta varten" (Experimental apparatus for NMR-imaging of a moving object), by Pohjonen, Licenciate thesis, Helsinki Technical University, Department of Technical Physics, 1984, pp. 39-40.
The contrast in images formed by these imaging sequences is discussed in "Signal intensity in fast NMR imaging with short repetition times", by Buxton, Fisel, Chien and Brady, J. Magn. Reson. vol. 83, pp. 576-585, 1989. The utilization of magnetization transfer phenomenon in MRI for generation of tissue contrast is also described in the U.S. Pat. No. 5,159,270.
The utilization of relaxation under the locking conditions in MRI has been described in U.S. Pat. No. 4,799,015 to Sepponen,.
In a selection of the imaging sequence for MR imaging one tends to maximize tissue contrast and resolution with as good signal-to-noise ratio as possible and with as short imaging time as possible. These requirements are in many respect contradictory. Tissue contrast is generated by suppressing signals emitted by tissues by an exploitation of relaxation processes. In the sequences used today in MRI, relaxation affects every tissues during the scanning process. The differences in relaxation in tissues may be enhanced by using contrast agents, such as Gd-DTPA compound, manufactured by Schering AG of Germany, and sold under the trademark Magnevist, or by using a magnetization transfer-phenomenon. Magnetization transfer-phenomenon has been described in U.S. Pat. No. 5,159,270 and "Magnetic Resonance Imaging", by Stark and Bradley, C. V. Mosby Company, St Louis, pp. 204-218, 1991. In addition the use of magnetization transfer in MRI has been reviewed in "Magnetization transfer contrast in magnetic resonance imaging", by Balaban and Ceckler, Magnetic Resonance Quarterly, Vol. 8, no. 2, pp. 116-137, 1992.
The contrast provided by known methods is limited by the strength of the polarizing magnetic field B.sub.o. To maximize the signal-to-noise ratio and scanning speed a relatively high strength polarizing magnetic field is generally utilized. Magnetization transfer (MT) and relaxation time based contrast, however, are better at low strengths of the polarizing magnetic field. Because of this, low strengths of the polarizing magnetic field should be used. By using known methods it is difficult if not impossible to get a high signal-to-noise ratio, a good contrast and short scanning time simultaneously. In practice one is forced to make compromises, which decrease the diagnostic value of the study.
The present invention describes a method which is capable of eliminating the drawbacks of the prior art and provide a high tissue contrast and a high signal-to-noise ratio within a short scanning time.